Digestive System, Nutrition and Excretion

Digestive sytem and Nutrition

Ascidians are filter feeders removing plankton from the water current pumped through the pharyngeal basket. The water current is produced by lateral cilia on the margins of the gill slits of the pharyngeal basket (Ruppert et al., 2004). A vast quantity of water is required to produce enough food. On average the equivalent of one body volume of water is pumped through and filtered every second (Ruppert et al., 2004). A mucus net is excreted by the endostyle, covering the pharyngeal basket lining, to trap suspended food particles within the water current (Ruppert et al., 2004). The frontal cilia on the pharyngeal lining transport the continuously secreted net from the endostyle to a more dorsal position (Pechanik, 2010). Upon reaching the dorsal midline of the pharynx, the mucus net is rolled into a threadlike cord by the dorsal lamina, it is then transferred into the esophagus (Ruppert et al., 2004).

Photograph of the basal section of a zooid after being in a microplastic solution. The esophagus and intestine are white due to the microplastic beads within them.

The net is a meshwork containing a complex iodoprotein and regular openings of ~0.5µm (Ruppert et al., 2004). Ascidians are able to stop feeding by closing the inhalant siphon, by halting the ciliary beating, or by stopping the production of mucus by the endostyle (Ruppert et al., 2004).

Once through the pharynx, the digestive tract is a U-shape (Ruppert et al., 2004). The esophagus leads in a dorsal direction to the stomach, at the base of the U. The stomach is the site of extra-cellular digestion and is lined with secretory cells (Ruppert et al., 2004). The intestine then ascends from the stomach, terminating with a rectum and an anus. The intestine forms faeces and is likely to be the site of absorption (Ruppert et al., 2004). The anus discharges faeces into the atrium for expulsion through the exhalant siphon.

A network of tubules forms the pyloric gland, this network covers the outer wall of the intestine (Ruppert et al., 2004). Collecting canals from the pyloric gland open into the intestine near its site of attachment to the stomach (Harrison and Ruppert, 1991). The products secreted by the pyloric gland may be digestive enzymes and substances involved in pH regulation (Ruppert et al., 2004). The gland also aids in the removal of toxins from the blood (Ruppert et al., 2004) and is a glycogen store (Gaill, 1980).

Excretion

Tunicates lack nephridia and most lack excretory organs entirely and probably rely on the diffusion across tissues to dispose of nitrogenous wastes (Ruppert et al., 2004). However, other metabolic byproducts are stored internally and released when the zooid dies. This is known as storage secretion (Ruppert et al., 2004). The byproducts may be stored in nephrocytes, which then accumulate in the various tissues within the body, they may also be secreted by epethelia into extracellular compartments such as the pericardial cavity or other specialised structures (Ruppert et al., 2004). It is not clear whether E. diaphanis has any specialised excretory structures.